The present invention relates to methods and apparatus for detecting defects in masks used in semiconductor processing. More particularly, the present invention relates to apparatus and methods for detecting defects in phase shift masks.
Fabrication of semiconductor wafers typically relies on photolithography to produce circuit patterns on layers of a wafer. The wafer is coated with a xe2x80x9cphotoresistxe2x80x9d. Light is then transmitted through the mask and imaged onto the wafer. Photoresist is a material which is sensitive to light. A negative photoresist cures or hardens when exposed to light, so that the unexposed areas can be washed away. For example, in one system, ultraviolet light is used to expose a portion of the photoresist layer. A positive photoresist reacts in the opposite manner, the exposed regions can be washed away. The photoresist that is left acts as a mask, so that materials may be deposited in the areas not covered by photoresist to thereby form patterns on the wafer. The photoresist is then removed.
Designers and manufacturers constantly strive to develop smaller devices from the wafers, recognizing that circuits with smaller features generally have greater speeds and increased yield (numbers of usable chips produced from a standard semiconductor wafer). Photolithography equipment manufacturers have generally employed equipment using progressively smaller wavelengths to a current size below 193 nm in order to achieve smaller feature sizes. However, as the size of the circuit features decrease, physical limits such as the convergence between the wavelength of the light used to create the photoresist mask and the wafer feature sizes present obstacles to further reduction in feature size using the same semiconductor fabrication equipment.
Designers of such equipment have discovered that a phase shift mask (PSM) will allow the patterning of smaller features, even as the feature size approaches the wavelengths of the light used to create the photoresist pattern from the PSM. In some cases the use of a PSM may decrease the minimum feature size by a factor of two. With PSM, the mask no longer looks like the design shapes. Instead, the PSM contains shapes which cause the design shapes to appear as a result of constructive and destructive interference of light passing through the PSM. Alternating phase shift masks generally use an etching technique to etch a small depression into the mask. Light passing through the depression experiences a phase shift relative to the unetched areas, creating sharper images at the wafer. While the overall process uses particular design rules for minimum feature size, a PSM allows circuits with more aggressive critical dimensions to be consistently built using existing lithography tools.
Due to their importance in decreasing feature size while using existing equipment, semiconductor and semiconductor equipment manufacturers are highly motivated to detect PSM defects. Given the small size of the features and the volumes of wafers to be produced from a mask, it is essential that defects be detected in the masks to either enable repairs, where appropriate, of the mask or discard unsalvageable masks prior to production.
Conventional inspection techniques such as optical methods work well in identifying defects of typical chrome on glass masks. These defects include the placement of chrome in unintended places and the absence of chrome portions where desired. A conventional chrome on glass mask is shown in FIGS. 1A and 1B. FIG. 1A shows a cross section with chrome sections 102 and 104 deposited on transparent layer 106. A typical material for layer 106 is quartz, due to its ability to transmit light. FIG. 1B is a top view of the photomask showing a typical defect 108. Conventional optical inspection techniques work well in identifying such defects because the amplitude of the light transmitted through the defect is directly affected, i.e., the absence of the normally opaque chrome section allows light to be transmitted and detected in a location where such detection is unexpected. Contaminants on the glass can be identified by using either transmitted or reflected light or a combination of the two. The defects directly affect the amplitude of light passing through and reflected from the mask and are amenable to measurement by the above referenced conventional techniques.
Phase shift defects, however, present unusual problems. Imaging of phase objects and detection of phase defects typically requires special imaging methods to convert the phase information into intensity differences at the imaging detector. Numerous methods have been proposed to accomplish this including the Zernike phase contrast, differential interference contrast (DIC), differential phase contrast (DPC), defocused imaging, and interferometric techniques. Most of these methods involve changing the phase delay of the optical wavefront in the pupil plane of the imaging system in a way that will produce the greatest intensity effect at the detector for a given phase defect or phase object. The optimum method greatly depends on the phase shifts present in the object. In biological samples weak phase shifts need to be imaged. In phase shift masks for photolithography strong phase shifters are used. As a result, a sensitive defect detection system for phase defects on phase shift masks must detect weak phase objects in the presence of strong phase and amplitude objects. Of particular interest in the design is the response of a system to phase edges. Another important aspect for automated photomask inspection systems is whether the system response to phase objects is isotropic in the plane of the object. An anisotropic response as yielded by the DIC or a Nomarski technique or the linear DPC technique may be acceptable for visual inspection but complicates automated inspection.
For the foregoing reasons, there is a need for improved methods and apparatus capable of detecting phase shift mask defects in the presence of both weak and strong phase shifts.
To achieve the foregoing, the present invention provides apparatus and methods for detecting phase defects.
The invention relies generally on scanner type imaging systems and detects phase defects on photomasks by their modification of the light passing through the mask. Much of the detail provided in the specification for practicing the invention is given for scanners. Those skilled in the art, having the benefit of the details provided in this specification, will appreciate that the invention may also be implemented on projector type imaging systems. Specifically, the invention relies on the modification of the phase of the wavefront at the pupil plane of the optical imaging system using defocus in conjunction with the differential detection of the image intensity from different segments of a detector.
In order to enhance the signals generated from phase defects, the invention in several embodiments provides a defocused light inspection beam by setting the focus of the scanning beam to a level above or below the photomask subject to inspection, resulting in a defocused beam. The light from the scanning beam reflected or transmitted by the photomask is collected by a detector split into at least two portions, each generating a signal. The detector is typically positioned at or near the pupil plane. A resulting differential signal produced from the signals generated at each of the two detector portions is used to determine whether a defect in the photomask is present, in one embodiment, by generating an image from the resulting signal.
In one aspect, the first portion of the detector is a circular region and the second portion is an annular region outside the first region.
In one aspect the invention provides a method for detecting phase defects in a semiconductor processing photomask by using a complex amplitude plate such as a Zernike phase shift plate or similar phase plate. The beam generated using a Zernike phase plate or similar phase plate is used to derive a differential signal from the two portions of the detector to produce the resulting signal indicating whether phase defects are present.
In another aspect the invention provides a method of detecting phase defects in a first mode using a defocused light beam and a differential signal from the two portions of the detector. In a second mode, detecting pattern defects occurs by adjusting the focus of the optical inspection beam to the surface of the photomask and by summing the signals from the first and second portions of the detector to generate the resulting signal to produce at-focus, normal imaging for Cr defects.